Cold-cathode tube operating apparatus

ABSTRACT

The present invention can provide a cold cathode tube lighting apparatus capable of causing no pulsating current, intermittent oscillation, or flicker-off even under light load, stably keeping a cold cathode tube lighting, safely tuning on the same, and the like. The present invention is characterized by supplying a rectangular wave voltage Vs to a series resonance circuit  12  and by driving the cold cathode tube  8  with an output of the series resonance circuit. The series resonance circuit has a constant that makes a maximum output voltage with respect to a predetermined tube current value exceed a tube voltage V L  of the cold cathode tube when the cold cathode tube is turned on and in an operating load range of the cold cathode tube of negative resistance characteristic. A control circuit  15  controls a cold cathode tube current I L  to a predetermined value while the cold cathode tube is in a lit state. At black start of the cold cathode tube, the control circuit prevents a tube voltage of the cold cathode tube from exceeding a predetermined voltage until the cold cathode tube is turned on.

This application is a 371 application (National Stage entry) ofinternational patent application No. PCT/JP03/03137 filed on Mar. 17,2003, which claims foreign priority to JP-2002-087956 filed on Mar. 27,2002.

TECHNICAL FIELD

The present invention relates to a cold cathode tube lighting apparatus,and particularly, to a cold cathode tube lighting apparatus with aseries resonance circuit for lighting a cold cathode tube.

BACKGROUND TECHNOLOGY

A conventional cold cathode tube lighting apparatus 50 shown in FIG. 1typically consists of a chopper circuit 51 to control a cold cathodetube current I_(L) to a predetermined value, a parallel resonancecircuit 52 composed of a transformer and a capacitor, a ballastcapacitor C5 to stabilize discharge, a tube current sensing circuit 14,and a control circuit 53 to control a power supply period for thechopper circuit 51.

To reduce the size of the transformer as small as possible, a turn ration thereof is set to n={V_((STRIKE))}/(2πV_(IN(DC)), where V_((STRIKE))(see FIG. 2) is a lighting start voltage of a cold cathode tube 8. It isusual to set a maximum output voltage of the secondary side of thetransformer to V_((STRIKE)).

DISCLOSURE OF INVENTION

When the conventional apparatus 50 turns on the cold cathode tube 8, theballast capacitor C5 bears a voltage of I_(L)/(j·ω·Cb) with respect to acold cathode tube current I_(L). Accordingly, a secondary output voltageV_(to)={(I_(L)/(j·ω·Cb))²+(I_(L)·R_(L))²}^(1/2) of the voltage necessaryfor maintaining discharge under light load exceeds the maximum outputvoltage V_((STRIKE)) that transformer can actually provide from thesecondary side thereof. This results in causing a pulsating current,intermittent oscillation, or flicker-off, to destabilize a lit state ofthe cold cathode tube 8.

The present invention has been made in consideration of theabove-mentioned problem and provides a cold cathode tube lightingapparatus capable of causing no pulsating current, intermittentoscillation, or flicker-off even under light load, stably keeping a coldcathode tube lighting, and safely tuning on the same.

The present invention also provides a cold cathode tube lightingapparatus having no risk of badly affecting peripheral devices when acold cathode tube is detached.

The present invention also provides a cold cathode tube lightingapparatus capable of stably keeping a cold cathode tube lighting evenwith a low source voltage.

The present invention also provides a cold cathode tube lightingapparatus capable of keeping a cold cathode tube lighting with a powersource being in a highly efficient state.

According to a first technical aspect of the present invention to solvethe above-mentioned problem, there is provided a cold cathode tubelighting apparatus having a rectangular wave voltage generating circuitto generate a rectangular wave voltage from a direct current inputvoltage, a series resonance circuit having a resonance inductance, afirst resonance capacitor, a cold cathode tube of negative resistancecharacteristic, and a constant, to convert the rectangular wave voltageinto a sine wave voltage, the constant being set to make a maximumoutput voltage for a predetermined tube current value exceed a tubevoltage of the cold cathode tube at the start of lighting the coldcathode tube and in an operating load range of the cold cathode tube ofnegative resistance characteristic, a cold cathode tube voltage sensingcircuit, a cold cathode tube current sensing circuit, and a controlcircuit to control a cold cathode tube current to a predetermined valueaccording to an output of the cold cathode tube current sensing circuitwhile the cold cathode tube is in a lit state and prevent a cold cathodetube voltage from exceeding a predetermined voltage according to anoutput of the cold cathode tube voltage sensing circuit until the coldcathode tube is lit when the cold cathode tube is black-started.

According to a second technical aspect of the present invention to solvethe above-mentioned problem, the control circuit in the cold cathodetube lighting apparatus prevents, if the cold cathode tube is detached,a cold cathode tube voltage from exceeding a predetermined voltage andstops the operation of the rectangular wave generating circuit after apredetermined time.

According to a third technical aspect of the present invention to solvethe above-mentioned problem, the series resonance circuit in the coldcathode tube lighting apparatus is additionally provided with a step-uptransformer.

According to a fourth technical aspect of the present invention to solvethe above-mentioned problem, there is provided a cold cathode tubelighting apparatus having a rectangular wave voltage generating circuitto generate a rectangular wave voltage from a direct current inputvoltage, a series resonance circuit having a resonance inductance, afirst resonance capacitor, a second resonance capacitor, a cold cathodetube of negative resistance characteristic, and a constant, to convertthe rectangular wave voltage into a sine wave voltage, the constantbeing set to make a maximum output voltage for a predetermined tubecurrent value exceed a tube voltage of the cold cathode tube at thestart of lighting the cold cathode tube and in an operating load rangeof the cold cathode tube of negative resistance characteristic, a coldcathode tube voltage sensing circuit, a cold cathode tube currentsensing circuit, and a control circuit to control a cold cathode tubecurrent to a predetermined value according to an output of the coldcathode tube current sensing circuit while the cold cathode tube is in alit state and prevent a cold cathode tube voltage from exceeding apredetermined voltage according to an output of the cold cathode tubevoltage sensing circuit until the cold cathode tube is lit when the coldcathode tube is black-started.

According to a fifth technical aspect of the present invention to solvethe above-mentioned problem, the control circuit in the cold cathodetube lighting apparatus prevents, if the cold cathode tube is detached,a cold cathode tube voltage from exceeding a predetermined voltage andstops the operation of the rectangular wave generating circuit after apredetermined time.

According to a sixth technical aspect of the present invention to solvethe above-mentioned problem, the series resonance circuit in the coldcathode tube lighting apparatus is additionally provided with a step-uptransformer.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view showing circuits in an example 50 of a conventionalcold cathode tube lighting apparatus;

FIG. 2 is a graph showing a tube current-tube voltage characteristic anda tube current-tube impedance characteristic of a cold cathode tube;

FIG. 3 is a view showing circuits in a cold cathode tube lightingapparatus 1 according to a first embodiment of the present invention;

FIG. 4 is a view showing a series resonance circuit picked up from FIG.3;

FIG. 5 is a graph showing an output voltage characteristic of the seriesresonance circuit 12 shown in FIG. 4;

FIG. 6 is a graph showing an output voltage of the series resonancecircuit when a cold cathode tube is black-started in the cold cathodetube lighting apparatus 1 according to the first embodiment of thepresent invention;

FIG. 7 is a graph showing an output voltage of the series resonancecircuit when a cold cathode tube is detached in the cold cathode tubelighting apparatus 1 according to the first embodiment of the presentinvention;

FIG. 8 is a view showing a cold cathode tube lighting apparatus 2according to a second embodiment of the present invention;

FIG. 9 is a view showing a cold cathode tube lighting apparatus 3according to a third embodiment of the present invention;

FIG. 10 is a view showing a cold cathode tube lighting apparatus 4according to a fourth embodiment of the present invention;

FIG. 11 is a view showing a cold cathode tube lighting apparatus 5according to a fifth embodiment of the present invention;

FIG. 12 is a view showing a cold cathode tube lighting apparatus 6according to a sixth embodiment of the present invention;

FIG. 13 is a view showing a cold cathode tube lighting apparatus 7according to a seventh embodiment of the present invention;

FIG. 14 is a view showing an example of a tube voltage sensing circuit13;

FIG. 15(A) is a view showing an example waveform of a tube current I_(L)under time sharing control; and

FIG. 15(B) is a view showing an example waveform of a time sharingsignal St.

BEST MODE OF IMPLEMENTATION

The embodiments of the present invention will be explained withreference to the drawings.

First Embodiment

FIG. 3 shows a cold cathode tube lighting apparatus 1 according to thefirst embodiment and a cold cathode tube 8 connected to the apparatus.The cold cathode tube lighting apparatus 1 consists of a rectangularwave voltage generating circuit 11, a series resonance circuit 12, atube voltage sensing circuit 13, a tube current sensing circuit 14, anda control circuit 15.

The rectangular wave voltage generating circuit 11 is connected to ordisconnected from a direct current input voltage V_(IN(DC)) and outputsa positive-negative-symmetrical rectangular wave voltage Vs. Therectangular wave voltage Vs changes its pulse width in response to adrive signal Sd. The rectangular wave voltage generating circuit 11 maybe of a known one, and therefore, no internal structure thereof will beshown or explained.

The series resonance circuit 12 consists of a resonance inductance L1, afirst resonance capacitor C1, and the cold cathode tube 8. Among them,the resonance inductance L1 has an end connected to the rectangular wavevoltage generating circuit 11, to receive the rectangular wave voltageVs from the rectangular wave voltage generating circuit 11. The otherend of the resonance inductance L1 is connected to an end of the firstresonance capacitor C1. The other end of the first resonance capacitorC1 is earthed. A node between the resonance inductance L1 and the firstresonance capacitor C1 is connected to a high-voltage terminal 17 of thecold cathode tube 8. A low-voltage terminal 18 thereof is earthedthrough the tube current sensing circuit 14.

The tube current sensing circuit 14 may also be a known one. Forexample, the tube current sensing circuit 14 of the conventionalapparatus shown in FIG. 2 may be employed. The tube voltage sensingcircuit 13 may also be a known one. An example thereof is shown in FIG.14. An input terminal of the tube voltage sensing circuit 13 isconnected to the high-voltage terminal 17 of the cold cathode tube 8.The tube voltage sensing circuit 13 detects a voltage at the node, i.e.,a tube voltage V_(L) of the cold cathode tube 8 and outputs a voltagedetected signal Sv.

The control circuit 15 has error amplifiers 19 and 20, a triangular waveoscillating circuit 22, a shut-down circuit 23, a timer circuit 24, aPWM control circuit 25, a drive circuit 26, and the like. The controlcircuit 15 also has a regulator, a start circuit, and the like, whichare not directly related to the operation of the present invention, andtherefore, are not shown or explained. “PWM” is pulse width modulation.Among them, the error amplifier 19 amplifies a difference between afeedback signal Sf from the tube current sensing circuit 14 and areference voltage Vr1 and outputs a current error signal Sie. The othererror amplifier 20 amplifies a difference between the voltage detectedsignal Sv from the tube voltage sensing circuit 13 and a referencevoltage Vr2 and outputs a voltage error signal Sve.

The current error signal Sie and voltage error signal Sve are suppliedto inverting input terminals (−) of the PWM control circuit 25. Atriangular wave from the triangular wave oscillating circuit 22 issupplied to an in-phase input terminal (+) of the PWM control circuit25. According to the input signals, the PWM control circuit 25 outputs apulse width signal Sw. At this time, a larger one of the current errorsignal Sie and voltage error signal Sve is selected. Namely, when anexcessive voltage or current increases the error signal, the pulse widthsignal Sw is controlled to narrow the pulse width of the rectangularwave.

In addition to them, the PWM control circuit 25 receives an ON/OFFsignal Sp and a shut-down signal Ss. The ON/OFF signal Sp is a signal toturn on/off the cold cathode tube 8. It is set to a high level to turnon the cold cathode tube and a low level to turn off the same. Only whenthe signal Sp is at a high level, the pulse width signal Sw is provided.The shut-down signal Ss is provided by the shut-down circuit 23 toprotect circuits when the tube voltage V_(L) reaches an open protectivevoltage Vo (see FIGS. 6 and 7, a predetermined voltage set to beslightly higher than a lighting start voltage V_((STRIKE))). Uponreceiving the shut-down signal Ss, the PWM control circuit 25 stopsoutputting the pulse width signal Sw. As shown in FIGS. 6 and 7, theopen protective voltage Vo is a predetermined voltage set to be slightlyhigher than the lighting start voltage V_((STRIKE)).

The timer circuit 24 supplies an operation stop signal Sb to theshut-down circuit 23 during a delay period Td shown in FIG. 7. While theoperation stop signal Sb is being supplied, the shut-down circuit 23provides no shut-down signal Ss even if the tube voltage V_(L) reachesthe open protective voltage Vo.

The pulse width signal Sw from the PWM control circuit 25 is supplied tothe drive circuit 26. While the pulse width signal Sw is being supplied,the drive circuit 26 supplies a drive signal Sd to switching elements(not shown) of the rectangular wave voltage generating circuit 11.According to the drive signal Sd, the rectangular wave voltagegenerating circuit 11 generates the rectangular wave voltage Vs, and ifthe drive signal Sd is stopped, stops generating the rectangular wavevoltage Vs.

The drive circuit 26 also receives a time sharing signal St shown inFIG. 15(B). The time sharing signal St is to temporarily turn off thecold cathode tube 8 at predetermined intervals. During a high-levelperiod Th of the time sharing signal St, the drive signal Sd is notprovided. Accordingly, the tube current I_(L) of the cold cathode tube 8is pulse-modulated in response to the time sharing signal St as shown inFIG. 15(A). Namely, the tube current is intermittently driven. Moreprecisely, the tube current I_(L) has a frequency of, for example, 50kHz and the time sharing signal St has a frequency of, for example, 200Hz (a period of 5 ms). Human eyes are unable to recognize theintermittence of the cold cathode tube 8 operating at 200 Hz andrecognize that the intensity of the cold cathode tube 8 is averaged andlowered. During the period Th in which the tube is turned off accordingto the time sharing signal St, no power is supplied, and therefore,there is no efficacy deterioration.

Operation of the cold cathode tube lighting apparatus according to thisembodiment will be explained.

Initially, the rectangular wave voltage generating circuit 11 is in astandby state with the direct current input voltage V_(IN(DC)) beingsupplied thereto. When a power source Vcc is supplied to the controlcircuit 15, the internal regulator and start circuit start to put thecontrol circuit 15 in a standby state. When the ON/OFF signal Sp is setto a high level, the rectangular wave voltage generating circuit 11starts to receive the drive signal Sd and output the rectangular wavevoltage Vs.

When driven around the resonance frequency of the series resonancecircuit 12, the rectangular wave voltage Vs is shaped by the seriesresonance circuit 12 substantially into a sinusoidal waveform, which isapplied to the high-voltage terminal 17 of the cold cathode tube 8. Atthis time, the control circuit 15 controls the duty of the rectangularwave voltage Vs, and therefore, the output voltage of the seriesresonance circuit 12 is kept at the open protective voltage Vo (FIG. 6).When black start is conducted, the cold cathode tube 8 is turned onafter a black start period Th of 0.5 to 2 seconds (FIG. 6). When normalstart is conducted, the cold cathode tube 8 is turned on more quickly.

Once the cold cathode tube 8 is lit, the tube current I_(L) thereof isdetected by the tube current sensing circuit 14. To maintain the tubecurrent I_(L) at a predetermined value, the control circuit 15 controlsthe duty of the rectangular wave voltage Vs. In addition, the tubevoltage V_(L) is detected by the tube voltage sensing circuit 13. If thetube voltage V_(L) exceeds the open protective voltage Vo due to somereason, the control circuit 15 controls the duty of the rectangular wavevoltage Vs in such a way as to narrow the pulse width of the rectangularwave voltage Vs.

If the cold cathode tube 8 is detached, the rectangular wave voltage Vsis stopped after the delay period Td (FIG. 7). Accordingly, there is norisk of badly affecting peripheral devices.

Advantages in driving the cold cathode tube 8 with the series resonancecircuit 12 will further be explained. FIG. 4 shows the series resonancecircuit 12 picked up from the circuits shown in FIG. 3. An outputvoltage Vout of this circuit varies depending on load resistance Rout.As shown in FIG. 5, it generates a higher voltage as the load resistanceRout is increased. In FIG. 5, R_(L1) to R_(L3) represent impedancevalues of the cold cathode tube 8 and have a relationship ofR_(L3)>R_(L2)>R_(L1).

If stable discharge is maintained in a load range where the cold cathodetube 8 demonstrates a negative resistance characteristic, the tubevoltage V_(L) and tube impedance R_(L) of the cold cathode tube 8increase if the tube current I_(L) of the cold cathode tube 8 decreases.

In this case, the load resistance Rout shown in FIG. 3 corresponds tothe tube impedance R_(L). Increasing the tube impedance R_(L)corresponds to increasing the load resistance value Rout of the seriesresonance circuit 12 shown in FIG. 3. As a result, the output voltageVout of the series resonance circuit 12 also increases as mentionedabove.

This characteristic well matches with the characteristic of the coldcathode tube 8 that the impedance R_(L) increases as the tube currentI_(L) decreases. If the tube current I_(L) decreases to increase theimpedance R_(L), a higher voltage is needed to maintain a lit state. Inthis case, the output voltage Vout of the series resonance circuit 12increases in response to the increase in the impedance R_(L), to supplythe voltage necessary for keeping the lit state of the cold cathode tube8. Namely, employing the series resonance circuit realizes a circuitstructure matching with the impedance characteristic of the cold cathodetube 8.

With respect to a predetermined tube current value, the maximum outputvoltage of the series resonance circuit 12 must be greater than the tubevoltage V_(L) to maintain discharge. Otherwise, no stabilized lit stateis maintained even if a feedback circuit and the like are employed forstabilization. Then, like the conventional lighting apparatus 50 of FIG.1, a pulsating current, intermittent oscillation, or flicker-off willoccur.

To avoid this, the present invention sets a constant of the seriesresonance circuit 12 so that the maximum output voltage of the seriesresonance circuit 12 with respect to a predetermined tube current valueexceeds the tube voltage V_(L) of the cold cathode tube 8 at the time oflighting the cold cathode tube and in an operating load range of thecold cathode tube of negative resistance characteristic.

An example of this will be explained with reference to FIGS. 2 and 5. Itis assumed that the rectangular wave voltage Vs has a constant drivefrequency of fl. If the tube current is I_(L1), the cold cathode tube 8needs a voltage V_(L1) to stably maintain discharge with the loadcurrent. At this time, the load of the series resonance circuit 12 is animpedance Rout equivalent to a cold cathode tube impedance R_(L1) atthis time. An output voltage Vout1 of the series resonance circuitI_(L2) is set to satisfy Vout1≧V_(L1). Similarly, it is set to satisfyVout2≧V_(L2) with a tube current of I_(L2) and to satisfy Vout3≧V_(L3)with a tube current of I_(L3).

If the maximum output voltage of the series resonance circuit 12 withrespect to a predetermined tube current value is greater than the tubevoltage V_(L) of the cold cathode tube 8, the lighting state of the coldcathode tube 8 is stabilized. Accordingly, the tube current sensingcircuit 14 and control circuit 15 are used to control the pulse width ofthe rectangular wave voltage Vs, thereby adjusting a power supplyquantity from the rectangular wave voltage generating circuit 11 andobtaining a required tube voltage V_(L) and tube current I_(L).

The series resonance circuit however, generates a high voltage when noload is present. Accordingly, if the cold cathode tube 8 is detached,peripheral devices will badly be affected and the reliability of thelighting apparatus itself will deteriorate.

On the other hand, the cold cathode tube lighting apparatus 1 mustcontinuously provide the lighting start voltage V_((STRIKE)) until thecold cathode tube 8 is turned on in black start of the cold cathode tube8.

To solve these problems, the tube voltage V_(L) is prevented fromexceeding the open protective voltage Vo until the cold cathode tube 8is lit at black start of the cold cathode tube 8 as shown in FIG. 6.This prevents an unnecessary high voltage and supplies a sufficientvoltage to stably turn on the cold cathode tube at black start.

When the cold cathode tube 8 is detached, the tube voltage V_(L) isprevented from exceeding the open protective voltage Vo, and theoperation of the rectangular wave voltage generating circuit 11 isstopped after the delay period Td to provide protection for the tubedetached state, as shown in FIG. 7. More precisely, as explained above,the timer circuit 24 stops the operation of the shutdown circuit 23during the delay period Td, to thereby continuously apply a voltage tothe cold cathode tube 8. If there is an overvoltage after the delayperiod Td (no lighting), the shut-down circuits 23 operates to stopapplying the voltage. The delay period Td is a period set to be slightlylonger than the black start period Th of the cold cathode tube 8. Theblack start period Th is about 0.5 to 2 seconds, and therefore, thedelay period Td is set to be slightly longer than that, for example, 2.5seconds.

Second Embodiment

FIG. 8 shows a cold cathode tube lighting apparatus 2 according to thesecond embodiment.

This cold cathode tube lighting apparatus 2 inserts a second resonancecapacitor C2 between a rectangular wave voltage generating circuit 11and a resonance inductance L1. This is different from the cold cathodetube lighting apparatus 1 of the first embodiment. Inserting the secondresonance capacitor C2 results in stably maintaining a lit state with alower input voltage V_(IN(DC)) in a range where the tube impedance of acold cathode tube is low.

Third Embodiment

FIG. 9 shows a cold cathode tube lighting apparatus 3 according to thethird embodiment.

This embodiment arranges a step-up transformer 28 between a firstresonance capacitor C1 and a cold cathode tube 8. This results in stablymaintaining a lit state even with a further reduced input voltageV_(IN(DC)).

Fourth Embodiment

FIG. 10 shows a cold cathode tube lighting apparatus 4 according to thefourth embodiment.

This embodiment arranges a step-up transformer 28 after a resonanceinductance L1, and after the step-up transformer, a first resonancecapacitor C1. This arrangement can also maintain a stable lit state witha lower input voltage V_(IN(DC)).

Fifth Embodiment

FIG. 11 shows a cold cathode tube lighting apparatus 5 according to thefifth embodiment.

This embodiment arranges a first resonance capacitor C1 after a leakagetransformer 29. A leakage inductance LL of the leakage transformer 29 isused as a resonance inductance of a series resonance circuit. Due tothis, there is no need of preparing the separate resonance inductance L1of the first embodiment. This results in reducing the number of parts,cost, and parts space.

Sixth Embodiment

FIG. 12 shows a cold cathode tube lighting apparatus 6 according to thesixth embodiment.

A display panel of, for example, a notebook personal computer isprovided with a reflection panel around a cold cathode tube 8. Thereflection panel is usually earthed, to form a parasitic capacitor Cx.FIG. 14 additionally considers this parasitic capacitor Cx ascapacitance of a resonance capacitor. This results in more correctlysetting a constant. Additionally considering a parasitic capacitor of awiring board will further improve the correctness of computation.

Seventh Embodiment

FIG. 13 shows a cold cathode tube lighting apparatus 7 according to theseventh embodiment.

This embodiment forms a first resonance capacitor from two capacitors C3and C4. These two capacitors C3 and C4 are also used as voltage dividingcapacitors of a tube voltage sensing circuit 13. This embodiment,therefore, reduces the number of parts, cost, and parts space.

The embodiments control each the pulse width of the rectangular wavevoltage Vs. The present invention is not limited to such control. Forexample, the present invention can control the frequency (period) of therectangular wave voltage Vs. In this case, a resonance circuit constantis set such that the drive frequency of the rectangular wave voltage Vsis always higher than the resonance frequency of the series resonancecircuit 12. Then, an increase in the frequency of the rectangular wavevoltage Vs lowers the output voltage Vout of the series resonancecircuit, and a decrease in the frequency of the rectangular wave voltageVs provides an opposite result The present invention can control boththe pulse width and frequency of the rectangular wave voltage Vs.

According to the first technical aspect of the present invention, arectangular wave voltage is supplied to the series resonance circuitwhose output drives a cold cathode tube. The series resonance circuithas a constant that is set to make a maximum output voltage with respectto a given tube current value higher than a tube voltage of the coldcathode tube at the start of lighting the cold cathode tube and in anoperating load range of the cold cathode tube of negative resistancecharacteristic. While the cold cathode tube is being lit, the controlcircuit controls a cold cathode tube current to a predetermined value,and at black start of the cold cathode tube, controls a tube voltage tobe higher than a predetermined voltage until the cold cathode tube isturned on.

As a result, no pulsating current, intermittent oscillation, orflicker-off occurs even under low load, and the cold cathode tube isstably lit and is maintained at the lit state. In addition, the lightingof the cold cathode tube is safely started.

According to the second technical aspect of the present invention, thecontrol circuit further prevents, if the cold cathode tube is detached,a cold cathode tube voltage from exceeding a predetermined voltage andstops the operation of the rectangular wave generating circuit after apredetermined time.

As a result, there will be no risk of badly affecting peripheral deviceseven if the cold cathode tube is detached.

According to the third technical aspect of the present invention, theseries resonance circuit is additionally provided with a step-uptransformer to step up an output voltage.

This results in stably turning on the cold cathode tube and maintaininga lit state thereof even with a low source voltage.

According to the fourth technical aspect of the present invention, asecond resonance capacitor is arranged before a resonance inductance, tostably maintain a lit state with a lower input voltage V_(IN(DC)) inparticular in a range where the tube impedance of the cold cathode tubeis low.

According to the fifth technical aspect of the present invention, thecontrol circuit operates like the control circuit of claim 2 when thecold cathode tube is detached.

As a result, a lit state is stably maintained with a lower input voltageV_(IN(DC)) in a range where the tube impedance of the cold cathode tubeis low.

According to the sixth technical aspect of the present invention, anoutput voltage is increased in the cold cathode tube lighting apparatusof any one of claims 4 and 5, like the invention of claim 3.

This results in achieving the same effect as the invention of claim 3.In addition, a lit state is stably maintained with a lower input voltageV_(IN(DC)) in a range where the tube impedance of the cold cathode tubeis low.

1. A cold cathode tube lighting apparatus comprising: a rectangular wavevoltage generating circuit to generate a positive-negative-symmetricalrectangular wave voltage from a direct current input voltage; a seriesresonance circuit having a resonance inductance connected in series witha parallel connection of a first resonance capacitor and a cold cathodetube, to convert the rectangular wave voltage into a sinusoidal wavevoltage, a constant of the series resonance circuit being set to make amaximum output voltage for a predetermined cold cathode tube currentvalue exceed a tube voltage of the cold cathode tube at the start oflighting the cold cathode tube and in an operating load range of thecold cathode tube of negative resistance characteristic; a cold cathodetube voltage sensing circuit to detect a tube voltage of the coldcathode tube and output a voltage detected signal; a cold cathode tubecurrent sensing circuit to detect a tube current of the cold cathodetube and output a current detected signal; and a control circuitcontrolling the duty of the rectangular wave voltage, the controlcircuit including: a timer circuit outputting an operation stop signalduring a delay period that is longer than a black start period; ashut-down circuit outputting a shut-down signal in a case where thevoltage detected signal is above a predetermined voltage when the timercircuit is not supplying the operation stop signal; a first erroramplifier amplifying a difference between the current detected signaland a first reference voltage and outputting a current error signal; asecond error amplifier amplifying a difference between the voltagedetected signal and a second reference voltage and outputting a voltageerror signal; a triangular wave oscillating circuit generating atriangular wave signal; a PWM control circuit comparing the currenterror signal and voltage error signal with the triangular wave signaland outputting a pulse width signal when the shut-down circuit is notsupplying the shut-down signal; and a drive circuit outputting a drivesignal while the PWM control circuit is supplying the pulse widthsignal, wherein the drive signal being outputted to the rectangular wavevoltage generating circuit while the cold cathode tube is in a lit stateso that a cold cathode tube current has a predetermined currentaccording to the current detected signal from the cold cathode tubecurrent sensing circuit, the drive signal being outputted to therectangular wave voltage generating circuit until the cold cathode tubeis lit at the time of black start of the cold cathode tube so that atube voltage of the cold cathode tube is set to be greater than alighting start voltage and smaller than an open protective voltageaccording to the voltage detected signal from the cold cathode tubevoltage sensing circuit, and the drive signal being output to therectangular wave voltage generating circuit when the cold cathode tubeis detached so that a tube voltage of the cold cathode tube is set to begreater than the lighting start voltage and lower than the openprotective voltage according to the voltage detected signal from thecold cathode tube voltage sensing circuit, and the drive signal to therectangular wave voltage generating circuit being stopped after thedelay period.
 2. The cold cathode tube lighting apparatus of claim 1,wherein the series resonance circuit further includes a second resonancecapacitor connected in series with the resonance inductance.
 3. The coldcathode tube lighting apparatus of claim 1 or 2, wherein the seriesresonance circuit further includes a step-up transformer connectedbetween the first resonance capacitor and the cold cathode tube.
 4. Thecold cathode tube lighting apparatus of claim 1 or 2, wherein the seriesresonance circuit further includes a step-up transformer connectedbetween the resonance inductance and the first capacitor.
 5. The coldcathode tube lighting apparatus of claim 4, wherein the step-uptransformer is a leakage transformer and the resonance inductance is aleakage inductance of the leakage transformer.
 6. The cold cathode tubelighting apparatus according to claim 1, wherein the first resonancecapacitor includes a parasitic capacitor around the cold cathode tube.7. The cold cathode tube lighting apparatus according to claim 1,wherein the first resonance capacitor is a voltage dividing capacitor ofthe tube voltage sensing circuit.